A radio treatment thereby simulator is employed to improve efficiency and accuracy of radio therapy planning, allowing more precise tumor localization and assessment of target volume. It enables exact simulation of the actual treatment plan. The X-ray simulator comprises a patient couch positioned between the horizontally extending arms of a C-shaped gantry that is mounted for rotation about a horizontal axis extending substantially along the patient supporting couch. An X-ray source and collimator on the end of one arm cooperate with a film holder on the other arm to enable X-ray and fluoroscopic analysis and study of the patient. The collimator includes a reticle having a set of cross hairs, the intersection of which forms a target point that is projected onto the patient's body substantially at the X-ray beam axis. The projection of the target point enables precisely located marks to be made upon the patient's body for use in analysis and assessment of tumor location and subsequent treatment. Precision of the actual treatment depends in part upon target point location. However, precision location of the target point formed by the collimator assembly cross hairs is difficult to achieve and subject to many errors which may include bending of the cantilevered support arm, tolerance in the mounting of the support arm or the gantry, and tolerance in mounting of the X-ray head itself to the support arm. A major error source is deflection of the support arm which, because it is generally constructed with an asymmetrical cross section, experiences different amounts of deflection at different positions of gantry rotation.
Errors in the positioning of the target point are minimized by manufacture of the gantry and support arm by making the structure sufficiently massive and rigid to bring the aiming errors caused by arm deflection and the like down to an acceptable level. Thus, in one high quality X-ray simulator, the isocentric accuracy, that is the location of the target point on or along the rotation (isocentric) axis in various positions of gantry rotation, has a tolerance of one millimeter in a direction transverse to the isocentric axis and of two millimeters in a direction along the isocentric axis. Nevertheless, if the length or weight of the arm should be increased as, for example, by employing an X-ray head or adjustable mounting that provides weight greater than design weight at the end of the support arm, deviation of the target point along the isocentric axis may be increased beyond acceptable limits. Accordingly, it is an object of the present invention to provide apparatus in which target point tolerances are significantly decreased.